1. Field of the Invention
The present invention relates to a magnetic memory device and a method of manufacturing the same and is applied to, e.g., a magnetic random access memory (MRAM).
2. Description of the Related Art
In recent years, MRAMs which use the magnetoresistance effect of ferromagnetic materials have received a great deal of attention as next-generation solid-state nonvolatile memories capable of implementing fast read/write, large capacity, and low power consumption. Especially, the interest in MRAMs has been growing since it was found that a magnetoresistance effect element having a ferromagnetic tunnel junction exhibited a high magnetoresistance change ratio.
The ferromagnetic tunnel junction basically has a three-layered structure including a free layer (magnetization free layer) whose magnetization direction easily changes depending on an external field, a pinned layer (magnetization fixed layer) which opposes the free layer and maintains a predetermined magnetization direction even upon receiving an external field, and a tunnel barrier layer (insulating layer) which is sandwiched between the free layer and the pinned layer. In the ferromagnetic tunnel junction, a current flows through the tunnel barrier layer. At this time, the resistance of the junction portion changes depending on the relative angle of the magnetization directions of the pinned layer and free layer. When the magnetization directions are parallel, the resistance takes a minimal value. When the magnetization directions are anti-parallel, the resistance takes a maximal value. The change in resistance is called a tunneling magnetoresistance effect (to be referred to as a TMR effect hereinafter). When a magnetic element having a ferromagnetic tunnel junction is actually used as a memory cell of an MRAM, the parallel and anti-parallel states (i.e., minimal and maximal values of resistance) of the magnetizations of the pinned layer and free layer are made to correspond to “0” and “1” of binary information, thereby storing information.
The magnetic information write operation is done by laying out a write interconnection near the memory cell and causing a current magnetic field generated upon supplying a current to reverse only the magnetization direction of the free layer. The magnetic information read operation is executed by supplying a sense current to the memory cell and detecting a change in resistance by the TMR effect. A magnetoresistance effect element using the above-described TMR effect will be referred to as a magnetic tunnel junction (MTJ) element hereinafter.
When MTJ elements are integrated to implement an MRAM of GBit class, a write current necessary for a write in an MTJ element increases. As an attempt to reduce the write current, a simple U-shaped yoke layer which is open to the MTJ element side is formed on the write interconnection (e.g., U.S. Pat. No. 6,661,688).
A conventional magnetic memory device having a U-shaped yoke layer on a write interconnection will be described in more detail with reference to FIGS. 1 and 2. Referring to FIGS. 1 and 2, yoke layers 11 and 12 formed on write interconnections are extracted.
As shown in FIGS. 1 and 2, both the two write interconnections have the U-shaped yoke layers 11 and 12, respectively. An MTJ element is arranged at the intersection between the write interconnections. When write currents I-11 and I-12 are supplied to the write interconnections having the yoke layers 11 and 12, magnetic fields 13 and 14 are generated by the Ampere's law. Since the yoke layers 11 and 12 prevent any leakage of the magnetic fields 13 and 14 around the write interconnections, the write efficiency increases, and the write currents decrease.
However, when a distance L1 between the write interconnections is short (e.g., L1≦2,000 Å), the expected write current reducing effect cannot be obtained because of the following factors.
At ends 16 of the yoke layers, a magnetic field M1 generated from the upper write interconnection and a magnetic field M2 generated from the lower write interconnection are directed in the same direction and strengthen each other to produce a synergistic effect. For this reason, the magnetic fields directed toward the MTJ element concentrate at the ends 16. On the other hand, at ends 17 of the yoke layers, the magnetic fields M1 and M2 are directed in opposite directions and weaken each other. The magnetic fields become weak so that a void region of the magnetic fields (magnetic field sink) directed toward the MTJ element is formed at the ends 17.
The write currents should decrease when the distance L1 between the interconnections is decreased by forming the yoke layers 11 and 12 on the interconnection layers. In fact, the effect of the yoke layers 11 and 12 cannot be obtained, and large write currents are still necessary.
In addition, the smaller the distance L1 between the write interconnections is (e.g., L1≦2,000 Å), the more conspicuous the increase in write current is. For this reason, the manufacturing process to decrease the distance L1 between the write interconnections to reduce the write currents is useless and only increases the manufacturing cost.
As described above, in the conventional magnetic memory device, when the distance between write interconnections is decreased, magnetic fields near the intersection between the write interconnections become nonuniform and decrease near the element. Hence, the is write currents increase.